The present invention relates to a marine propulsion apparatus having a double-propeller construction, and more specifically having a pair of propellers which can be driven in rotation in the opposite directions to both exert a forward thrust.
Apparatus of this general type is shown in FIG. 6 of U.S. Pat. No. 2,989,022 issued on June 20, 1961 and Japanese Patent Provisional Publication 60-259594 laid open on Dec. 21, 1985, for example. Such prior art apparatus requires two vertical driving shafts and means for dividing the driving power between the shafts. This increases the horizontal length and the weight of the apparatus, thereby lowering the propulsive efficiency.
In addition, with reference to FIG. 3 of the present application, which shows a part of the structure of the Japanese publication with a modification in accordance with the present invention, an oil seal 70 is provided directly between the inner and outer propeller shafts 5 and 7. The thickness of this seal increases the diameter of the outer shaft 7, resulting in an increased diameter of the propeller surrounding the outer shaft.
It is a general object of the invention to provide a compact and lightweight marine propelling apparatus having a pair of propellers.